In recent years, the amount of information communication has been steadily increasing. In response to this trend, for example, for devices such as IC cards and mobile telephones, communication at regions of higher frequencies, such as a microwave and a millimeter wave, has become active. For this reason, the market has been requiring a high-frequency circuit substrate that can be used in a high-frequency region and that further decreases the transmission delay and transmission loss.
The transmission velocity V, which relates to the transmission delay, and the transmission loss αd can be expressed by Formulae 1 and 2, respectively, shown below by using the specific inductive capacity εr and dielectric loss tangent tan δ of the substrate material.V∝1/√εr   Formula 1Δd∝f×√εr×tan δ   Formula 2,                where f denotes frequency.        
Formula 1 shows that it is desirable to use a material having a low specific inductive capacity εr to decrease the transmission delay, that is, to increase the transmission velocity V. Formula 2 shows that it is desirable that the specific inductive capacity εr and the dielectric loss tangent tan δ be low to decrease the transmission loss αd.
The types of material having a low specific inductive capacity and a low dielectric loss tangent, described above, include fluororesins such as polytetrafluoroethylene (PTFE), a tetrafluoroethylene-perfluoroalkylvinyl ether copolymer (PFA), a tetrafluoroethylene-hexafluoropropylene copolymer (FEP), an ethylene-tetrafluoroethylene copolymer (ETFE), and polyvinylidene fluoride (PVdF). Researchers and engineers have developed high-frequency circuit substrates in which a dielectric layer made of fluororesin is formed on a metal base material (conductor) (for example, Patent Literatures 1 and 2).